Nerve growth factor supports growth of rat skeletal myotubes in culture

The role of neurotrophins in muscle under physiological and pathological conditions

This review summarizes the various effects of neurotrophins in skeletal muscle and how these proteins act as potential regulators of development, maintenance, function, and regeneration of skeletal muscle fibers. Increasing evidence suggests that this family of neurotrophic factors not only modulates survival and function of innervating motoneurons and proprioceptive neurons but also development and differentiation of myoblasts and muscle fibers. Neurotrophins and neurotrophin receptors play a role in the coordination of muscle innervation and functional differentiation of neuromuscular junctions. However, neurotrophin receptors are also expressed in differentiating muscle cells, in particular at early developmental stages in myoblasts before they fuse. In adults with pathological conditions such as human degenerative and inflammatory muscle disorders, variations of neurotrophin expression are found, but the role of neurotrophins under such conditions is still not clear. The goal of this review is to provide a basis for a better understanding and future studies on the role of these factors under such pathological conditions and for treatment of human muscle diseases.

Signal-transduction networks and the regulation of muscle protein degradation

Protein degradation in muscle functions in maintaining normal physiological homeostasis and adapting to new homeostatic states, and is required for muscle wasting or atrophy in various pathological states. The interplay between protein synthesis and degradation to maintain homeostasis is complex and responds to a variety of autocrine and intercellular signals from neuronal inputs, hormones, cytokines, growth factors and other regulatory molecules. The intracellular events that connect extracellular signals to the molecular control of protein degradation are incompletely understood, but likely involve interacting signal-transduction networks rather than isolated pathways. We review some examples of signal-transduction systems that regulate protein degradation, including effectors of proteolysis inducing factor (PIF), insulin and insulin-like growth factor (IGF) and their receptors, and fibroblast growth factor (FGF) and its receptors.

Nerve growth factor (NGF) influences differentiation and proliferation of myogenic cells in vitro via TrKA

Classic studies have established that muscle cells exert trophic actions on neurons of the developing peripheral nervous system through the production of neurotrophins. For this reason neurotrophins are also known as 'target-derived factors'. During differentiation, muscle cells also express some neurotrophin receptors, such as the low-affinity p75 neurotrophin receptor, which binds all neurotrophins, and the high affinity tyrosine kinase receptor TrKA, nerve growth factor (NGF) transducing receptor. The functional roles of these receptors in muscle cells are still unclear and only fragmentary and controversial data are available regarding the responsiveness of muscle cells to NGF. The aim of the present study is to investigate the effects of NGF on cells of myogenic lineage. The rat myogenic cell line L6, primary cultures of adult human myoblasts, and the human rhabdomyosarcoma cell line TE-671 were used in this study. As expected, all the three cell types expressed NGF, p75 and TrKA. NGF was expressed by L6 and primary myoblasts following differentiation, but it was constitutively expressed at high levels in the TE-671 rhabdomyosarcoma cells. In L6 myoblasts, p75 receptor was expressed in myoblasts but not in myotubes early after plating; while some primary human myoblasts expressed it at all the time-points tested. Some fusiform cells of the TE-671 rhabdomyosarcoma cell line also expressed p75. TrKA was constitutively immunodetected in all the three cell lines, suggesting that these cells may respond to NGF. Addition of exogenous NGF increased the fusion rate of both primary and L6 myoblasts, as well as the proliferation of the slowly dividing primary myoblasts. Consistently, blocking the action of endogenously produced NGF with a specific neutralizing antibody decreased the percentage of fusion in both primary and L6 myoblasts. On the contrary, blocking the binding of NGF to p75 did not affect the percentage of fusion. Furthermore, neither exogenous NGF nor NGF- or p75-neutralizing antibodies appeared to affect the rhabdomyosarcoma cells, which have a high proliferation rate and do not fuse. Pharmacological inhibition of TrKA signal transduction with K252a (in the nM range) and tyrphostin AG879 (in the low microM range) resulted in a dramatic dose-dependent decrease in proliferation of all of the myogenic cell lines tested. Interestingly, this was especially evident in the rapidly dividing rhabdomyosarcoma cell line. The TrKA inhibitors also blocked fusion of L6 and primary myoblasts and induced morphological changes characterized by the flattening of the cells and a 'spider-like' rearrangement of the intermediate filaments in all three cell lines with some minor differences. A transfection study showed that p75-overexpressing L6 cells do not fuse and present changes in their morphology similar to the TrKA-inhibitors treated L6 cells. These data support the notion that NGF expression in skeletal muscle is not only associated with a classical target-derived neurotrophic function for peripheral nervous system neurons, but also with an autocrine action which affects the proliferation, fusion into myotubes, and cell morphology of developing myoblasts. The present data also suggest that these effects of NGF are mediated by TrKA receptors and that a sustained presence of NGF is needed for increase fusion into myotubes. Lastly, the dramatic anti-proliferative effect of TrKA inhibitors on myogenic cells, and especially on the TE-671 rhabdomyosarcoma cell line, suggests that pharmacological interference with NGF signal transduction could be effective in the control of these malignancies.

Regulation of the p75 neurotrophin receptor in a rat myogenic cell line (L6)

Neurotrophins are expressed in muscle cells both during development and postnatally. Furthermore, during development muscle cells express high levels of the common p75 neurotrophin receptor, which binds all neurotrophins. Only fragmentary and controversial data are available regarding the responsiveness of muscle cells to neurotrophins and the importance of low-affinity p75 receptor in muscle development. The present study investigates in vitro the immunocytochemical expression of p75 in a rat myogenic cell line (L6) at various time points and in response to different coating substrates as a first step in elucidating the regulation of p75 in muscle. We found that in L6 myoblasts, p75 is expressed only at very early stages of maturation and its levels of expression are regulated by the nature of the coating substrates. p75 expression decreases in cells growing on substrates more suitable for myoblast fusion into myotubes. Time course analysis indicates a reverse correlation between myoblast fusion into myotubes and the levels of p75 expression. Myotubes were always p75 negative. Substrates not suitable for the fusion process induced a prolonged presence of p75 in myoblasts with an increase of their apoptosis. We conclude that expression of p75, at least in this in vitro condition, is regulated by the stages of myoblast differentiation and the nature of the coating substrates. According to the observed time- and substrate-related evidences, future studies should investigate in vivo both the regulation of p75 in the myoblast fusion and the effects and the importance of neurotrophins binding during myoblast differentiation.

Discoordinate regulation of different K channels in cultured rat skeletal muscle by nerve growth factor

We investigated the effects of nerve growth factor (NGF) on expression of K+ channels in cultured skeletal muscle. The channels studied were (1) charybdotoxin (ChTx)-sensitive channels by using a polyclonal antibody raised in rabbits against ChTx, (2) Kv1.5 voltage-sensitive channels, and (3) apamin-sensitive (afterhyperpolarization) channels. Crude homogenates were prepared from cultures made from limb muscles of 1-2-day-old rat pups for identification of ChTx-sensitive and Kv1.5 channels by Western blotting techniques. Apamin-sensitive K+ channels were studied by measurement of specific [125I]-apamin binding by whole cell preparations. ChTx-sensitive channels display a fusion-related increase in expression, and NGF downregulates these channels in both myoblasts and myotubes. Voltage-dependent Kv1.5 channel expression is low in myoblasts and increases dramatically with fusion; NGF induces early expression of these channels and causes expression after fusion to increase even further. NGF downregulates apamin-sensitive channels. NGF increases the rate of fall of the action potential recorded intracellularly from single myotubes with intracellular microelectrodes. The results confirm and extend those of previous studies in showing a functional role for NGF in the regulation of membrane properties of skeletal muscle. Moreover, the findings demonstrate that the different K+ channels in this preparation are regulated in a discoordinate manner. The divergent effects of NGF on expression of different K+ channels, however, do not appear sufficient to explain the NGF-induced increase in the rate of fall of the action potential. The changes during the falling phase may rather be due to increases in channel properties or may result from an increased driving force on the membrane potential secondary to the NGF-induced hyperpolarization.

Immunocytochemical expression of human muscle cell p75 neurotrophin receptor is down-regulated by cyclic adenosine 3',5'-monophosphate

To investigate whether the immunocytochemical expression of low affinity neurotrophin receptor (p75) in human muscle is modulated by increased levels of intracellular cyclic adenosine 3',5'-monophosphate (cAMP), human cultured myogenic cells were treated with cAMP analogues dibutyryl cAMP (dbcAMP 0.5-1 mM) and 8-bromo cAMP (1 mM) or the adenylate cyclase activator forskolin (10-100 microM). Cultures were processed for indirect immunofluorescence microscopy using an anti-human p75 mAb. The treatment of cultured muscle cells with cAMP analogues or forskolin for two days induced a decrease of immunoreactivity for p75 and a reduction of both myotube formation and morphological cell differentiation. Removal of cAMP derivatives from the medium resulted in a return of immunoreactive cells to the levels of untreated controls. These data indicate that adenylate cyclase is involved in the regulation of human muscle p75.

Expression of the low-affinity NGF receptor during human muscle development, regeneration, and in tissue culture

The expression of the low-affinity NGF receptor (LNGFR) during human muscle development, regeneration, and in tissue culture was analyzed using a murine monoclonal antibody to human LNGFR (MAb ME 20.4). Muscle cells from 12-22-week fetuses stained strongly for LNGFR. In adult normal muscle, only intramuscular nerve endings showed immunoreactivity with MAb ME 20.4, but no staining was detected in muscle fibers. In Duchenne muscular dystrophy, immunohistologically demonstrable LNGFR was present in regenerating muscle fibers. In these fibers, LNGFR gene expression was also demonstrated at the transcriptional level using in situ hybridization with riboprobes coding with human LNGFR. Cultures from human fetal and adult muscle were studied by double label indirect immunofluorescence microscopy with MAb ME 20.4 and antisera against human fetal myosin. Most myosin-positive cells, both at the myoblast and myotube stages, displayed surface LNGFR immunostaining. In cells from fetal muscle, LNGFR was detected during the first 2 weeks in vitro, whereas in cells from adult muscle the expression of LNGFR was observed for up to 7 weeks. These findings suggest a potential involvement of LNGFR in human muscle development and regeneration.

Early signals in serum-induced increases in ouabain-sensitive Na(+)-K+ pump activity and in glucose transport in rat skeletal muscle are amiloride-sensitive

The acute effects of serum on sodium-potassium (Na(+)-K+) pump activity and glucose uptake in cultured rat skeletal muscle were studied. Addition of serum to myotubes in phosphate-buffered saline caused Na(+)-K+ pump activity (as measured by changes in the ouabain-sensitive component of both membrane potential and 86Rb uptake) to increase, with peak effects obtained after 30 min. The effect was blocked completely by treatment with amiloride, but not by tetrodotoxin, which blocks voltage-dependent Na+ channels. On transfer of myotubes to Na(+)-free, choline buffer, resting Na(+)-K+ pump activity decreased to about 10% of that in phosphate-buffered saline. Addition of regular serum, but not Na(+)-free serum, caused Na(+)-K+ pump activity to increase slightly. Similar results were obtained with serum on glucose uptake, the peak effect being reached within 15 min. Stimulation of glucose uptake by serum was partially reduced by amiloride and was not altered by tetrodotoxin. Removal of external Na+ also eliminated serum effects on glucose uptake. The results demonstrate that there are similar signals involving Na(+)-H+ exchange for serum-induced increases in Na(+)-K+ pump activity and glucose transport. The lack of complete blockade of serum-induced elevation of glucose transport suggests an additional, as yet undefined, intracellular signal for stimulation of this transport system.

The nerve growth factor receptor: a multicomponent system that mediates the actions of the neurotrophin family of proteins

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) are members of a family of structurally related proteins termed neurotrophins that promote the growth and survival of neurons in the central and peripheral nervous systems. Each of these proteins bind to at least two membrane receptors. One is the low affinity nerve growth factor receptor (p75), which binds each member of the neurotrophin family. The other is one of a family of tyrosine kinase receptors--trkA binds only NGF, the related trkB receptor binds BDNF and NT-3, and trkC binds NT-3 alone. This article reviews kinetic and biochemical information on p75 and its relationship to the trk gene products.

Tunicamycin reduces Na(+)-K(+)-pump expression in cultured skeletal muscle

The purpose of this study was to examine effects of tunicamycin (TM), which inhibits core glycosylation of the beta-subunit, on functional expression of the Na(+)-K+ pump in primary cultures of embryonic chick skeletal muscle. Measurements were made of specific-[3H]-ouabain binding, ouabain-sensitive 86Rb uptake, resting membrane potential (Em), and electrogenic pump contribution to Em (Ep) of single myotubes with intracellular microelectrodes. Growth of 4-6-day-old skeletal myotubes in the presence of TM (1 microgram/ml) for 21-24 hr reduced the number of Na(+)-K+ pumps to 60-90% of control. Na(+)-K+ pump activity, the level of resting Em and Ep were also reduced significantly by TM. In addition, TM completely blocked the hyperpolarization of Em induced in single myotubes by cooling to 10 degrees C and then re-warming to 37 degrees C. Effects of tunicamycin were compared with those of tetrodotoxin (TTX; 2 x 10(-7) M for 24 hr), which blocks voltage-dependent Na+ channels. TM produced significantly greater decreases in ouabain-binding and Em than did TTX, findings that indicate that reduced Na(+)-K+ pump expression was not exclusively secondary to decreased intracellular Na+, the primary regulator of pump synthesis in cultured muscle. Similarly, effects of TM were significantly greater than those of cycloheximide, which inhibits protein synthesis by 95%. These findings demonstrate that effects were not due to inhibition of protein synthesis. We conclude that glycosylation of the Na(+)-K+ pump beta-subunit is required for full physiological expression of pump activity in skeletal muscle.

Serum factor induces selective increase in Na-channel expression in cultured skeletal muscle

We have examined effects of horse serum (HS) and various fractions (1 million-1M, 300K, 100K, and 30K nominal molecular weight limit) obtained by ultrafiltration on expression of TTX-sensitive Na-channels and on activities of the Na-K pump and glucose transport systems in cultured myotubes obtained from 1-2-day-old neonatal rat pups. Five-day-old cells were transferred to serum-free medium with no hormone or growth factor supplements (DMEM) for 24 hr and then treated with the various serum fractions for 48 hr. Measurements were made of specific [3H]-saxitoxin (STX) binding, action potential properties, 86Rb-uptake and 2-deoxyglucose (2-DG) uptake. HS significantly increased all parameters compared to DMEM (increases in STX-binding, 69%; Rb-uptake, 65%; 2-DG uptake, 93%). Results of treatment with the separate fractions showed that the 300K fraction caused a significantly greater increase in STX-binding than either HS or the other fractions. In contrast, the increases in Rb and 2-DG uptakes induced by the different fractions were not different from that obtained with HS. We conclude that serum contains a factor that selectively increases expression of TTX-sensitive Na-channels in skeletal muscle.

Nerve growth factor and fibroblast growth factor influence post-fusion expression of Na-channels in cultured rat skeletal muscle

We have examined effects of nerve growth factor (NGF) and fibroblast growth factor (FGF) on the density of tetrodotoxin (TTX)-sensitive Na-channels in cultured rat skeletal muscle. Measurements were made of specific binding of [3H]saxitoxin (STX) and the frequency and rate of rise of spontaneously occurring action potentials, the physiological expression of Na-channel density. Cells were transferred to various growth conditions at 6 days in vitro, and measurements were made beginning 24 hr later. Both growth factors (GF) caused dose-related increases in Na-channels compared with myotubes maintained in normal, serum-supplemented growth medium. Maximum effects occurred with a concentration of NGF of 50 ng/ml and FGF of 15 ng/ml. Scatchard analysis of specific STX binding showed an increase in Bmax with no significant change in Kd. Similar increases occurred on rate of rise and frequency spontaneous action potential. Treatment of cultures with cycloheximide or actinomycin D, inhibitors of protein and RNA synthesis, completely prevented the increase in STX-binding induced by GF treatment. The results indicate that NGF and FGF have important effects on regulation of excitable cell gene products after differentiation.

Regulation by thyroid hormones of glucose transport in cultured rat myotubes

Primary cultures of skeletal muscle obtained from neonatal rats possess a saturable process for active glucose uptake, the myotubes having a relatively high affinity for the substrate with a Km of 1 mM. The expression of the glucose transport system was most apparent after fusion of single myoblasts to multinucleated myotubes [3-4 days in vitro (DIV)], at which time glucose uptake increased sharply to reach plateau values at about 6-8 DIV. Treatment of the cells at age 6 DIV with triiodothyronine or thyroxine caused a marked increase in glucose uptake beginning 4 h after treatment and reaching a maximum at 24 h. Thyroid hormone-induced increase in glucose uptake was not reduced by either tetrodotoxin or verapamil, thus indicating that the effect was not secondary to the ability of the hormone to increase contractile activity. The effect of thyroid hormones was eliminated completely by inhibition of protein synthesis. The results indicate that thyroid hormones play an important role in regulation of glucose transport in skeletal muscle.

Regulation of the sodium-potassium pump in cultured rat skeletal myotubes by intracellular sodium ions

The properties of the Na-K pump and some of the factors controlling its amount and function were studied in rat myotubes in culture. The number of Na-K pump sites was quantified by measuring the amount of [3H]ouabain bound to whole-cell preparations. Activity of the pump was determined by measurement of ouabain-sensitive 86Rb-uptake and component of membrane potential. Chronic treatment of myotubes with tetrodotoxin (TTX), which lowers [Na]i, decreased the number of Na-K pumps, the ouabain-sensitive 86Rb uptake, and the size of the electrogenic pump component of Em. In contrast, chronic treatment with either ouabain or veratridine, which increases [Na+]i, resulted in an elevated level of Na-K pump sites. This effect was blocked by inhibitors of protein synthesis. Neither rates of degradation nor affinity of pump sites in cells treated with TTX, veratridine, or ouabain differred from those in control cells. The number and activity of Na-K pump sites were unaffected by chronic elevation in [Ca]i or chronic depolarization. We conclude that alterations in the level in intracellular Na ions play the major role in regulation of Na-K pump synthesis in cultured mammalian skeletal muscle.

Characterization of the relation between sodium channels and electrical activity in cultured rat skeletal myotubes: regulatory aspects

The relation among sodium channel density, frequency of electrical activity and maximal rate of rise of the action potential was studied in developing and mature rat skeletal myotubes in culture. The number of tetrodotoxin (TTX)-sensitive Na-channels was determined by measurements of the amount of [3H]saxitoxin (STX) bound to the cultures, and electrical properties were recorded with intracellular microelectrodes. The EC50 for TTX-induced decreases in maximal STX-binding, frequency and rate of rise of action potentials was in the range 8-20 nM. The 3 variables increased in parallel with age in culture to reach peak values at age 7-8 days, and then decreased in parallel until 10-12 days in culture. The age-related increase in Na-channel density was decreased, but not abolished, by prevention of myoblast fusion. Treatment with the Ca2+ ionophore, A23187, down-regulated, and blockade of Ca-channels with verapamil up-regulated the number of Na-channels. Na-channel density was also increased by chronic treatment with TTX and elevated external [K+], which eliminated spontaneous electrical and contractile activity. Parallel effects were observed on frequency and rate of rise of action potentials. Up-regulation of Na-channels was prevented by simultaneous treatment of myotubes with inhibitors of protein synthesis. We conclude that electrical and mechanical activity of cultured myotubes regulate de novo synthesis of Na-channels through alterations in the level of cytosolic Ca2+.